Immersion-plated tin has been used as one of the alternative final finishes for printed wiring board (PWB) because it provides a uniform metallic coating for improved in-circuit-test (ICT) probe life, lubricity for press fit pins, and excellent solderability. Because of the strong affinity between copper and tin, inter-diffusion occurs spontaneously even at room temperature through bulk, grain boundary, and surface diffusion pathways, resulting in the formation of intermetallic compounds at the Sn/Cu interface as well as in the grain boundaries of tin-based coating layers. See C. Xu, et al., “Driving Force for the Formation of Sn Whiskers,” IEEE TRANSACTIONS ON ELECTRONICS PACKAGING MANUFACTURING, VOL. 28, NO. 1, January 2005. At room temperature, the primary intermetallic is the η phase (Cu6Sn5) and grain boundary diffusion is significantly faster than bulk diffusion. See B. Z. Lee and D. N. Lee, “Spontaneous Growth Mechanism of Tin Whiskers,” Acta Mater., vol. 46, pp. 3701-3714, 1998. This results in irregular growth of Cu6Sn5 in the grain boundaries of the Sn deposit. Cu diffusion into the grain boundaries of tin deposit combined with intermetallic compound formation creates a compressive stress within the tin deposit. This compressive stress increases with time, and in the presence of surface defects or strain mismatch, creates conditions conducive to tin's breaking through the oxide layer and forming a whisker. See K. N. Tu, “Irreversible Processes of Spontaneous Whisker Growth in Bimetallic Cu—Sn Thin-Film Reactions” Phys. Rev. B, vol. 49, pp. 2030-2034, 1994. Tin whiskers pose a major potential for catastrophic electrical short circuit failures between fine pitch circuits in high reliability systems such as heart pacemakers, spacecraft, or military weapons and radars. See F. W. Verdi, “Electroplated Tin and Tin Whiskers in Lead Free Electronics,” American Competitiveness Institute, November 2004.
The formation of intermetallic compounds (both η phase and ε (Cu3Sn) phase) consumes the free tin in the coating that is essential for good solderability. Thus, to ensure sufficient useable “free” tin at assembly, the minimum immersion tin deposit thickness of 1 micrometer is specified by IPC-4554. See IPC-4554 “Specification for Immersion Tin Plating for Printed Circuit Boards,” 2007, IPC Bannockburn, Ill. As the soldering temperature increases with the use of lead-free solders, some OEMs even ask for a minimum of 1.2 micrometer.